Gunning devices that project a material onto a target substrate for producing or repairing of refractory linings are generally known. Two widely used gunning methods for fabricating and repairing refractory linings are known as the gunnite-type and shotcrete-type gunning methods. Unlike other casting methods, these gunning methods require no framework for casting refractory linings and allow for easy application even on irregular shapes or where frameworking is difficult to construct. Accordingly, gunning methods have been widely used in fabricating and repairing refractory linings, particularly, in furnaces such as a blast furnace, hot stove, electric furnace, converter, ladle, tundish, basic oxygen furnace and reheating furnace.
In a gunnite method, a dry powdery material to be “gunned” is pneumatically fed through a transporting hose to a nozzle assembly where water is added to produce a wet, highly viscous gunning material with good adhesive properties. The gunning material is projected through the nozzle assembly so that the material adheres and cures on the furnace wall portion, whereby a refractory furnace lining is fabricated or repaired. The gunnite application method requires no premixing of material with water and can therefore be carried out rapidly and on short notice and clean-up of equipment is minimal. An additional advantage over other methods of fabricating or repairing furnace linings include not having to use a lining mold, thereby enabling cost reduction and improving working efficiency and enables the repair of both hot and cold furnace linings. However, one disadvantage of the gunnite method is that it is difficult to completely wet and thoroughly mix the material and water stream as it is transported through the application gunning lance, pipe or nozzle. This is particularly true for short (less than about 5 feet) gunning pipes. In these situations, a lack of thoroughness in mixing results in less than optimum and desirable applied mass homogeneity and density, an increase in material waste due to rebounding aggregate and poor mass adhesion and often excessive material pipe drip. Additionally, when a directional change in the flow of the gunning material is required, the material tends to exit the nozzle in a “split” non-homogenous stream where part of the stream is very dry while the other part is overly wet, a phenomenon that is independent of any attempted water control. A problem associated with an overly dry or poorly wet gunning material that is gunned onto the object target, is that a portion of the material does not adhere to the substrate and causes a loss of deflected particles (known as “rebound”) which lowers the adhesion percentage of the gunning material to the furnace wall, thus affecting the quality and durability of a refractory furnace mass. To overcome the problems associated with nozzle gunning methods, shotcrete-gunning methods were developed.
Shotcrete gunning methods produce refractories having a more uniform quality and better physical properties than obtained by the gunnite method and generally are used for producing high density, monolithic structures. In the shotcrete method, a gunning material is produced by mixing a dry material with water in a separate mixing device prior to delivery to a gunning device. The dry powdery material is pre-wet with water in a mixer and then pumped by a delivery pump through a transfer hose to a gunning device which projects the gunning material to a target using compressed air. Usually, a setting agent is added to the gunning material at the nozzle prior to the gunning material being projected onto a furnace wall structure.
The shotcrete gunning method is not without its attendant drawbacks, however, in that it is necessary to mix the dry material with water in a separate vessel until a suitable consistency is obtained. Thus, a shotcrete gunning material is mixed before it is supplied by the delivery pump to a gunning device requiring additional equipment, e.g., mixer and delivery systems, and manpower, when compared with the nozzle gunning method. Moreover, it is important to accurately control the amount of water to the gunning material in the shotcrete gunning method to maintain the proper consistency. As a result, skill on the part of the shotcrete-gunning operator is required to maintain the correct amount of water for a desirable composition. If too little water is used, blocking or premature hardening of the gunning material may occur in the pump or delivery hose. Conversely, if an excessive amount of water is used, there can occur separation of aggregates of coarse particles and fine powder which is contained in the gunning material to be sprayed causing uneven and poor quality refractory layers.
An additional disadvantage of the “shotcrete” method is the logistics of the mixer and pump. A certain amount of gunning material remains in the delivery hose and nozzle creating a waste of material and increased manpower costs for the emptying and cleaning of equipment.
Furthermore, unlike the gunnite application method, which can be employed in hot applications to repair furnace walls at elevated temperature (e.g., above 2000 degrees Fahrenheit), attempts at using the shotcrete gunning method for repairing refractories at high temperatures have not been very successful.
The foregoing illustrates limitations known to exist in present refractory coating methods and devices. Thus it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly an alternative apparatus for the gunning of a material is provided including the features more fully disclosed hereinafter.